Artificial joints, in particular knee joints, for orthoses or prostheses have an upper attachment part and a lower attachment part, which are connected to each other via a joint device. In the case of a knee joint, the upper attachment part has seats for a thigh stump or a thigh rail, whereas the lower attachment part has a lower leg socket or a lower leg rail. In the simplest case, the upper attachment part is connected to the lower attachment part pivotably by a monoaxial joint. It is only in exceptional cases that such an arrangement is sufficient to ensure the desired result, for example a supporting action when used in an orthosis, or a natural gait pattern when used in a prosthesis.
To ensure that the different requirements during the various phases of a step or during other actions are satisfied or supported in a way that is as natural as possible, resistance devices are made available that provide a flexion resistance or an extension resistance. By means of the flexion resistance, it is possible to establish how easily the lower attachment part can pivot relative to the upper attachment part in the direction of flexion. In a knee joint, therefore, the flexion resistance is used to establish how easily the lower leg socket or the lower leg rail swings back in relation to the thigh socket or the thigh rail when a force is applied. The extension resistance brakes the forward movement of the lower leg socket or of the lower leg rail and can form an extension limit stop. In other types of joints, for example the hip joint or the ankle joint, these observations apply correspondingly to the kinematic relationships.
With adjustable resistance devices, it is possible to adapt the respective flexion and/or extension resistance to the user of the prosthetic or orthotic device or to different walking or movement situations, so as to be able to provide a suitable resistance under changing conditions.
DE 10 2008 008 284 A1 discloses an orthopedic knee joint with an upper part and, arranged pivotably on the latter, a lower part which is assigned several sensors, for example a flexion angle sensor, an acceleration sensor, an inclination sensor and/or a force sensor. The extension stop is adjusted according to the sensor data that are determined.
DE 10 2006 021 802 A1 describes a control system of a passive prosthetic knee joint with adjustable damping in the direction of flexion, for adaptation of a prosthetic device having upper attachment means and a connector element to an artificial foot. The adaptation is made to climbing stairs, wherein a low-moment lifting of the prosthetic foot is detected, and the flexion damping in a lifting phase is lowered to below a level that is suitable for walking on the flat. The flexion damping can be increased depending on the change in the knee angle and depending on the axial force acting on the lower leg.
DE 10 2007 053 389 A1 describes a method and an appliance for controlling an orthopedic joint of a lower extremity with at least one degree of freedom, having an adjustable actuator by which an orthopedic device, comprising upper means of attachment to a limb and an orthopedic joint arranged in an articulated manner distally from the attachment means, is adapted to walking situations that deviate from walking on the flat. Several parameters of the orthopedic device are detected via sensors, the detected parameters are compared with criteria that have been established on the basis of several parameters and/or parameter profiles and are stored in a computer unit, and a criterion is selected that is suitable on the basis of the detected parameters or parameter profiles. Flexion resistances, movement ranges, drive forces and/or the profiles thereof are established in accordance with the selected criterion, in order to control special functions that deviate from walking on the flat. A tilt angle of a part of the orthopedic device in space and/or a profile of a change in tilt angle of a part of the orthopedic device can be used as parameter.
EP 1 237 513 B1 describes a prosthesis or orthosis with a control device and, coupled thereto, a sensor which detects an inclination angle relative to a fixed line of a part connected to a joint. On the basis of the inclination angle data, the movement properties of the joint are changed, i.e. the joint is braked or released.
It has proven useful that knee joints offer a high degree of resistance in the stance phase during walking or also during standing, wherein the joint is not completely blocked. In this case, bending of the joint during standing is prevented by the fact that the force vector lies in front of the joint axis and thus forces the joint to the extension limit stop.
A non-locking of the joint in the standing situation has the advantage that the user still has possible ways of intervening in the joint movement. For example, should he be standing on stairs and lose his balance, a locked joint would cause him to fall without any control, whereas he is still able to bend a joint with a high resistance by means of the stump force. He can thus minimize the consequences of a fall or avoid falling altogether. The resistance also makes it easier to maneuver the joint in confined spaces and to sit down. Sitting down is made easier by the fact that the resistance provides support against the joint giving way too quickly. The resistance applied against the joint giving way too quickly would otherwise have to be applied by the side not fitted with a prosthesis, i.e. the healthy leg.
If a prosthesis or orthosis user is in a seated position, the situation can arise whereby the flexion resistance and/or the extension resistance is so high that comfortable sitting is not possible. While sitting, it is often necessary and convenient to have free mobility of the joint, such that the small movements made while sitting can be performed without problems.